The present invention relates to a method for treating untreated water containing hydrogen sulfide (H.sub.2 S). Untreated water containing H.sub.2 S is found in well, surface and process waters used for both municipal potable and industrial uses, and concentrated brine solutions such as discharge streams from a reverse osmosis facility. The method of the invention involves converting the alkalinity (HCO.sub.3.sup.-) in the water to carbon dioxide (CO.sub.2), and then converting the H.sub.2 S to sulfate (SO.sub.4.sup.-2) ions. The resulting water may then be treated according to need to adjust the pH and/or hardness.
In a number of geographical areas, both municipal drinking water treatment plants and industrial plants are fed with water which contains H.sub.2 S. The presence of H.sub.2 S in the water imparts a "rotten egg odor" and can lead to the formation of elemental sulfur which affects turbidity. In H.sub.2 S laden waters treated with reverse osmosis, the H.sub.2 S will pass into the concentrated brine reject stream. The H.sub.2 S must be removed before discharging the brine to a surface water body.
The customary method of eliminating the H.sub.2 S from the source water is to adjust the pH to 5.5-6.0 with a strong mineral acid such as sulfuric acid. After pH adjustment, the water is degassified by forced draft degassifiers, sometimes followed by a gas scrubber to clean the gasses prior to their release to the atmosphere. This degassification/scrubbing process is inefficient because H.sub.2 S emissions and resulting odor problems inevitably occur. In addition degassification saturates the water with oxygen, resulting in downstream corrosion problems in the metal distribution piping, e.g., copper pipe. More specifically, scattered incidences of copper pipes developing holes frequently occur. Furthermore, corrosion of metal and concrete structures within the vicinity of degassification results in reduced life of the buildings.
Degassification therefore requires removal of oxygen from the water to prevent corrosion. Common methods for removing oxygen include treating the water with sodium sulfite (Na.sub.2 SO.sub.3) or hydrazine (N.sub.2 H.sub.4). Another oxygen removing method is vacuum degassification. However, all such oxygen removal procedures result in either unacceptable water quality or excessively high costs.
In order to avoid the presence of oxygen, the H.sub.2 S can be oxidized with a suitable oxidizing agent such as chlorine.
In the reaction of H.sub.2 S with chlorine, two chemical reaction paths may result, one of which causes problems in the water treatment and distribution system. The chemical reaction of H.sub.2 S and chlorine can proceed as follows: EQU H.sub.2 S+Cl.sub.2 .fwdarw.2HCl+S.degree. (1) EQU H.sub.2 S+4Cl.sub.2 +4H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 +8HCl (2)
The formation of elemental sulfur as shown in Equation (1) creates turbidity in the water and fouls downstream equipment. The removal and disposal of the sulfur results in high capital and operating costs. On the other hand, the reaction path of equation (2) shows that all of the H.sub.2 S is oxidized to sulfate (SO.sub.4.sup.-2), which remains in solution as a harmless anion.
The problem facing water treatment facilities is how to eliminate the formation of elemental sulfur produced in accordance with Equation (1). This has proven to be very difficult, and chlorine induced oxidation of H.sub.2 S has therefore been largely limited to situations where the elemental sulfur may remain in the water, such as in irrigation systems (where the sulfur is discharged along with the water itself) or in raw water feeds to water treatment plants (where filtration or other methods are then used to remove the elemental sulfur).
It would therefore be of a significant advance in the treatment of water to remove H.sub.2 S if the reaction represented by equation (1) is substantially reduced or eliminated so that little if any elemental sulfur is formed.